This research is broadly concerned with the question of why there is such great diversity of lipids in biomembranes and whether this diversity is due to differences in structure of the different lipid bilayers. The specific focus of this proposal is on continued development of methods that yield accurate enough structural information to distinguish between lipid bilayers and on application of these methods initially to lecithin bilayers in a variety of phases including the biologically relevant fully hydrated L-alpha phase. Our proposed structural studies are logically separated into two parts (i) average structure and (ii) fluctuations around average structure, which together provide a more complete picture of biological systems than studies of average structure alone. (i) Average structural quantities include area/lipid at the aqueous interface, thickness of the hydrocarbon region and lipid molecular volume. We will use our previously developed method of preparing oriented lecithin samples to determine the tilt angle of hydrocarbon chains in the subgel phase as well as other structural quantities such as unit cell dimensions. For the L-alpha phase both oriented samples and unoriented samples in varying mixtures of PVP polymer will be employed to determine the headgroup spacing. Together with our recently complete structure of the gel phase and measured differences in specific volume, this will yield the average structure of the L-alpha phase. Also, the question of change in bilayer structure with dehydration will be addressed. (ii) Our present work has revealed extensive diffuse scattering in the wide-angle region of the gel phase. We propose to extend these studies to the L-alpha and subgel phases. By comparing the diffuse scattering from lecithins of different chain length, estimates of the relative amounts of the diffuse scattering due to fluctuations from the tails versus the heads will be made. Model calculations and Monte Carlo simulations will be performed to obtain correlation functions to calculate diffuse scattering that will then be tested against our scattering data. If these theoretical studies pass this test, then they will provide more detailed information about fluctuations, as well as average structure, than can be extracted from our x-ray data alone.